如何在.net中使用SmtpClient发送邮件

我有以下代码不起作用：

public static void SendMail(string content, string title, List<string> address)
{
   SmtpClient client = new SmtpClient(Server,Port);
   client.Port = Port;
   client.Host = Server;
   client.EnableSsl = false;
   client.Timeout = 10000;
   client.DeliveryMethod = SmtpDeliveryMethod.Network;
   client.UseDefaultCredentials = false;
   client.Credentials = new System.Net.NetworkCredential(Username, Password);
   foreach(string to in address)
   {
      MailMessage mm = new MailMessage(From, to, title, content);
      mm.BodyEncoding = UTF8Encoding.UTF8;
      mm.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
      client.Send(mm);
   }
   client.Dispose();
}

我得到以下错误：

邮箱不可用。服务器的响应是：您必须提供您的用户名和密码才能通过此服务发送邮件

您可以看到我正在传递用户名和密码。为什么我仍然有这个问题？

MailMessage mail = new MailMessage();
        mail.To.Add(textBox1.Text);
        mail.From = new MailAddress("Yourgmailid");
        mail.Subject = "Email using Gmail";
        string Body = "Hi, this mail is to test sending mail" +
                      "using Gmail in ASP.NET";
        mail.Body = Body;
        mail.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com"; 
        smtp.Credentials = new System.Net.NetworkCredential
             ("Yourgmailid, "Password");
        smtp.EnableSsl = true;
        smtp.Send(mail);